Input should not be checked for native actions

[Lingxian Kong]

steps to regenerate:

1. run execution using the workflow https://github.com/openstack/mistral-extra/blob/master/examples/v2/openstack/nova_actions.yaml
2. error occured with traceback:
2015-06-29 03:00:02.433 13695 ERROR mistral.engine.default_engine [-] Failed to handle action execution result [id=234f7e23-a57f-4c5c-aadf-1708c9545f2b]: Invalid input [name=nova.servers_find, class=NoneType, unexpected=[u'status', u'id']]
Traceback (most recent call last):
  File "/vagrant/mistral_dev/mistral/engine/default_engine.py", line 237, in on_action_complete
    self._on_task_state_change(task_ex, wf_ex, action_ex)
  File "/vagrant/mistral_dev/mistral/engine/default_engine.py", line 181, in _on_task_state_change
    self._dispatch_workflow_commands(wf_ex, cmds)
  File "/vagrant/mistral_dev/mistral/engine/default_engine.py", line 358, in _dispatch_workflow_commands
    task_handler.run_new_task(cmd)
  File "/vagrant/mistral_dev/mistral/engine/task_handler.py", line 99, in run_new_task
    _run_existing_task(task_ex, task_spec, wf_spec)
  File "/vagrant/mistral_dev/mistral/engine/task_handler.py", line 61, in _run_existing_task
    task_ex.in_context
  File "/vagrant/mistral_dev/mistral/engine/task_handler.py", line 194, in _get_input_dictionaries
    ctx
  File "/vagrant/mistral_dev/mistral/engine/task_handler.py", line 208, in _get_workflow_or_action_input
    ctx
  File "/vagrant/mistral_dev/mistral/engine/task_handler.py", line 283, in _get_action_input
    wf_spec
  File "/vagrant/mistral_dev/mistral/engine/action_handler.py", line 96, in get_action_input
    e_utils.validate_input(action_def, input_dict)
  File "/vagrant/mistral_dev/mistral/engine/utils.py", line 56, in validate_input
    msg % tuple(msg_props)
InputException: Invalid input [name=nova.servers_find, class=NoneType, unexpected=[u'status', u'id']]

[Lingxian Kong (kong)]

https://review.openstack.org/196578